Process for preparing an undrawn, low birefringence polyamide yarn



Dec. 13, 1966 G. PITZL 3,291,880

PROCESS FOR PREPARING AN UNDRAWN, LOW BIREFRINGENCE POLYAMIDE YARN FiledDec. 23, 1964 l I D N EN YARN TEMP. c, AT POINT OF STEAMING I V GILBERTPITZL Maw ATTORNEY United States Patent Office 3,291,880 Patented Dec.13, 1966 3,291,880 PRGCESS FOR PREPARING AN UNDRAWN, LQW BIREFRINGENCEPGLYAMIDE YARN Gilbert Pitzl, Chattanooga, Team, assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed Dec. 23, 1964, Ser. No. 420,654 6 Claims.(Cl. 264176) This invention relates to an improved process forproduction of polyamide filaments and yarns.

Commercially available nylon yarn is usually produced by melt-spinningpolyamide filaments, by winding the undrawn yarn into a package andsubsequently drawing the yarn on a drawtwister or draw-winder. It hasbeen found highly desirable to subject the filaments, after quenchingand prior to winding, to a steam treatment to insure satisfactorypackage formation in winding. Such a process is disclosed by Babcock inUS. 2,289,860. In this process the filaments, after quenching, aresubjected to heat and high humidity (including steam), preferably for aperiod of at least 0.04 second before Winding.

It has now been found that unexpected process and product advantages canbe gained by critical adjustment of the steaming process whereby thefilaments enter the steaming zone at a higher than normal filamenttemperature and in duration of steaming is greatly reduced. Inaccordance with the present invention, an undrawn polyamide yarn of lowbirefringence is produced by a process which comprises extruding amolten synthetic fiber-forming polyamide in the form of filaments;cooling the extruded filaments to a temperature sufiicient to at leastpartially solidify them, the said temperature being at least about 15 C.above the force-to-draw transition temperature of the filaments;applying steam to the said filaments for a period of no longer thanabout 0.02 second and winding the filaments into a package.

Yarns produced by this process exhibit a low orientation in the undrawnstate, as indicated by low birefringence, and consequently may be drawnat a high draw ratio to produce yarn of the same tenacity and elongationas conventional yarns prepared at a lower draw ratio. This means thatthe undrawn yarn denier may be in creased without effecting the finalyarn denier and the productivity of the spinning unit is therebyincreased. In addition it is found that yarns produced by the process ofthis invention dye more deeply under the same conditions than doconventionally processed yarns.

The invention will be more readily understood by reference to thedrawings.

FIGURE 1 is a schematic drawing showing the various steps of the processof the present invention.

FIGURE 2 is a series of curves referred to more specifically in ExampleI wherein yarn birefringence as ordinate is plotted against yarntemperature for four different periods of steaming.

The values for the force-to-draw transition temperatures, as reportedherein, are determined by measuring the force-to-draw at different yarntemperatures and plotting a curve of force-todraw vs. yarn temperature.The

temperature at which a definite break in the curve is observed is takenas the transition temperature for the particular yarn. Since theforce-to-draw transition temperature for nylon varies with the degree ofcrystallinity, orientation and moisture content, the force-to-draw isdetermined by passing yarn directly after quenching to a heated feedroll of 6.72 inches diameter, passing the yarn around the feed roll for16 turns to insure temperature equilibration, then passing the yarn to adraw roll and drawing to a 2.2 draw ratio.

The surface temperature of the filaments is determined with acompensating thermocouple arrangement in which tion.

one of a pair of thermocouples is placed in contact with the runningfilament and the other thermocouple is heated electronically until thetwo are in balance. A commercially available instrument manufactured bythe Hastings- Radist Co. is used in measuring the filament temperaturereported herein.

The term birefringence as used herein refers to the absolute differencein refractive indices' along and perpendicular to the axis of a filamentin an unswollen condi- The term birefringence as applied tomultifilament yarns or strands refers to the birefringence of thefilaments in those yarns or strands. The birefringence of the filamentsis determined from observation of representative filaments betweencrossed plane-polarizing elements (e.g., Nicol prisms) using a Soleilcompensator for accuracy. The method is described in detail by Heyn inTextile Research Journal 22, 513 (1952).

Example I Polyhexarnethylene adipamide having a relative viscosity of3637 is prepared in the conventional manner and melt extruded to form 34filaments. The filaments are quenched by cross fiow air following theprocedure of Heckert US. 2,273,105, dated February 17, 1942. Thefilaments, at various temperatures are passed at 1500 y.p.m. through astreamer for periods of 0.091, 0.018, 0.013 and 0.0044 second and thenwound, undrawn,into a package. The yarn is thereafter drawn to a denierof approximately 70. The process is schematically illustrated at FIGURE1 wherein filaments 1 freshly extruded from spinner-ct 2 pass intocooling chimney 3 where they are contacted by cross flow air 4.Convergence guide 5, adjustable in position to assist control offilament temperature as described hereinafter, leads the filament, atthe desired temperature out of chimney 3 and into steamer 6 where across fiow of steam 7 contacts the still hot filaments. The filaments inthe undrawn condition are thereafter passed by suitable guides 8 onto apackage 9. The duration of steaming is varied by using steamers ofvarious lengths, (i.e. 82, 16, 12 and 4 inches respectively to providethe various steaming periods at the yarn speed indicated). The 82-inchsteamer is of the type described in Babcock US. 2,289,860, dated July14, 1942. The shorter steamers are of the type described in co-pendingapplication S.N. 420,547 filed in the name of James C. Davis on the sameday as this application. The latter design is used for the shortersteamers in order to provide a less turbulent steam flow as is founddesirable when applying steam to the yarn at higher yarn temperatures toprevent variations in yarn denier. The yarn temperature as it enters thesteam zone is varied by changing the steamer location relative to thespinneret and by converging the filaments with an additional guide atvarious locations in the chimney (the rate of cooling is decreased byconverging the filaments). The results are indicated in the curves ofFIGURE 2, the various curves A, B, C and D representing results obtainedwith steaming periods of 0.09, 0.018, 0.013 and 0.0044 secondrespectively. It will be observed that a sharp decrease in birefringenceoccurs when steam is applied to yarns at a yarn temperature above aboutC. Furthermore, birefringence increases When the contact with steam islengthened. A period of contact of from about 0.001 second to about 0.09second is operable with a period of from about 0.004 second to about0.02 second being preferred. In Table 1 birefringence values for varioussteaming periods at a constant yarn temperature of 85 C. are tabulated.The force-to-draw transition temperature is determined, as previouslydescribed, for the various yarns and found to be 59 C. The yarn steamedfor 0.013 second at 85 C. is tested for dyeability using anthraquinoneblue SW and is found to dye 810 shades deeper than the conven- 3 tionalyarn, which is steamed for 0.091 second at 50 C., under the same dyeingconditions.

70-denier 34-filament yarns were prepared following the generalprocedure of Example I. The birefringence of the undrawn yarn is variedas shown in Table 2 by using different steamer exposure periods and yarntemperatures. The yarn with the higher birefringence is prepared bysteaming for 0.091 second, the distance between the steamer andspinneret being adjusted to give the yarn a temperature of 52 C. at thepoint of entry into the steamer. The yarn with the lower birefringenceis prepared by steaming for 0.018 second, the steamer being raised togive a yarn temperature of 85 C. as the yarn enters the steamer. Whenthe yarn is drawn it is found that the draw ratio of the yarn having thelower birefringence can be increased to 2.95 as compared to 2.86 for theother yarn while the denier and elongation of the yarn are substantiallyequivalent. This increase in draw ratio represents an increase inproductivity for the spinning machine of about 34%. Also, as shown inthe table, the coefiicient of variation of the yarn denier (CV) is lowerfor the yarn having the lower birefringence.

TABLE 2 Birefrin- Draw Elongation gence Ratio Denier Percent Coefiicientof variation of yarn denier along length of yarn As demonstrated above,the process of the present invention by producing a modest change inbirefringence permits a 3.15% increase in draw ratio and therefore inproductivity. Such an increase in the commercial production of manymillions of pounds of fiber per year will be readily apparent. Underoptimum conditions, an increase in spinning machine productivity, aftercorrections are made for differences in elongation and denier, of asmuch as 10-15% may be attained. To achieve a substantial improvement inproductivity, the surface temperature of the filament entering thesteaming zone should be at least C. above the forceto-draw transitiontemperature and preferably at least 25 C. above this temperature. Theupper temperature limit is not critical with regard to birefringence butis limited by the operability of the process. In general, temperaturesmore than 60 C. above the force-to-draw transition temperatures shouldnot be employed and the preferred range is 25-40 C. above thistemperature.

The surface temperature of the filaments may be adjusted to the desiredlevel by modifying the quenching conditions, e.g., by adjustment ofquenching air flow and/ or temperature, adjusting the position of thesteamer relative to the spinneret or by varying the point at which thefilaments are converged into a yarn prior to entering the steamer. Thepreferred procedure is to adjust the distance between the spinneret andthe steamer so that the yarn is at the proper temperature when it entersthe steaming zone. I

Further decreases in the birefringence of the undrawn yarn, andconsequently greater improvements in productivity may be achieved byreducing the duration of steaming below that taught by Babcock in US.2,289,860.

. This reduction in steaming time can only be done in 4- combinationwith the higher yarn temperatures indicated above, since the use of suchshort steaming times at the lower yarn temperatures normally employedleads to unsatisfactory package formation as indicated by Babcock.Surprisingly, however, short steaming times may be used at the higheryarn temperatures without encountering problems due to poor pack-ageformation. The duration of steaming is desirably held to no more thanabout 0.02 second. With extremely short steaming times, some difficultymay be encountered due to steam turbulence which results in variation infilament denier and, consequently, for optimum results, times in therange of about 0.004 to about 0.018 second are preferred. The steamingtemperature, i.e., the temperature of the atmosphere in which thefilaments are treated, is not highly critical but should be above C.Steam pressures in the range of 5-50 pounds are suitable.

In addition to the increased spinning machine productivity attainable bythe process of this invention, it is found that fabrics prepared fromyarns processed according to this invention dye substantially deeperunder equivalent conditions than conventional, split' process nylonyarns. This is a decided advantage, both in the ease of dyeing and inattaining deep shades which are not normally obtained with such fibers.Also, when the yarn is steamed with the apparatus described in copendingapplication S.N. 420,547, the uniformity of the yarn and hence thefabric is markedly improved.

The yarn of this invention may be prepared from any polyamide whichcrystallizes readily in the presence of heat and moisture. The preferredpolyamides are 6-6 and 6 nylon. Other suitable polyamides are disclosedin US. 2,071,253, US. 2,030,523, and US. 2,130,948. Polyamides whichhave a high force-to-draw transition temperature may of course bedifficult to process and thus require special conditions to achievesatisfactory operability.

Many equivalent modifications will be apparent to those skilled in theart from a reading of the above without a departure from the inventiveconcept.

What is claimed is:

1. A process for preparing an undrawn, low birefringence polyamide yarnwhich comprises (1) extruding a molten, synthetic fiber-formingpolyamide in the form of filaments, (2) cooling the said filaments to atemperature at least about 15 C. above the force-to-draw transitiontemperature of the said filaments and low enough to at least partiallysolidify the said filaments, (3) contacting the said filaments withsteam for a period no longer than about 0.02 second and (4) forwardingthe said filaments from the said contact with steam.

2. The process of claim 1 wherein the said filaments are cooled to atemperature above about 60 C. prior to application of steam.

3. The process of claim 2 wherein the said temperature above about 60 C.is between about 25-40 C. above the force-to-draw transition temperatureof the said filaments.

4. The process of claim 1 wherein the said filaments are contacted withsteam for a period of from about 0.001 second to about 0.09 second.

5. The process of claim 1 wherein the said filaments are contacted withsteam for a period of from about 0.004 second to about 0.02 second.

6. The process of claim 1 wherein the said filaments are forwarded towindup after contact with steam.

References Cited by the Examiner UNITED STATES PATENTS 2,289,860 7/1942Babcock.

FOREIGN PATENTS 900,009 7/1962 Great Britain.

ROBERT F. WHITE, Primary Examiner. J. H. WOO, Assistant Examiner.

1. A PROCESS FOR PREPARING AN UNDRAWN, LOW BIREFRINGENCE POLYAMIDE YARNWHICH COMPRISES (1) EXTRUDING A MOLTEN, SYNTHETIC FIBER-FORMINGPOLYAMIDE IN THE FORM OF FILAMENTS, (2) COOLING SAID FILAMENTS TO ATEMPERATURE AT LEAST ABOUT 15* C. ABOVE THE FORCE-TO-DRAW TRANSITIONTEMPERATURE OF THE SAID FILAMENTS, AND HOW ENOUGH TO